In the electronic field there are a number of uses for very thin bodies of single crystalline silicon. For example, in the article of J. A. Oakes entitled, "A Pressure Sensor for Automotive Application," published in the Proceedings of Third International Conference on Automotive Electronics, October, 1981, pages 143-149, there is described a pressure sensor formed of single crystalline silicon and having a thin diaphragm region. In order to form the thin silicon bodies, such as the thin diaphragm region of the pressure sensor of J. A. Oakes, it has been the practice to etch a thicker silicon body down to the desired thickness.
Heretofore, an anisotropic etching technique has been used to form the bodies of silicon since such etching technique selectively etches more slowly along certain crystallographic planes, such as the &lt;111&gt; plane, than along other planes, such as the &lt;100&gt; and &lt;110&gt; planes, in etchants with sufficiently high pH values. Typical etchants include KOH, NaOH, LiOH, CsOH, NH.sub.4 OH, ethylenediamine pyrocatechol, and hydrazine. In addition to the crystal orientation selectivity of these etchants, a sufficiently high constant positive bias applied between a silicon body and an electrode both immersed in the etchant results in a current flow which causes the formation of a silicon dioxide layer on the surface of the body. The silicon dioxide layer resists etching and thus effectively prevents etching of the silicon body. This phenomena is referred to as "electrochemical etch stop." The bias required to passivate the surface of the silicon body is denoted as the "passivating potential (voltage)."
The electrochemical etch stop has been utilized in forming the bodies of silicon by starting with a body having a region of n-type conductivity adjacent a region of p-type conductivity which form a p-n junction. The n-type region is formed of the thickness desired for the thin body. The starting body is placed in the chemical etchant with the surface of the p-type region being exposed for etching, and a constant voltage is applied between the n-type conductivity region of the starting body and an electrode in the chemical etchant which is spaced from the starting body. This constant voltage reverse biases the p-n junction so that ideally no current flows into the p-type and n-type regions. The p-type region is anisotropically etched away by the chemical etchant until it is completely removed. After the p-type region is completely removed (etched away), the n-type region is exposed to the etchant. The n-type region now acts as a relatively low resistance resistor with a voltage thereacross which gives rise to a current therethrough. This current flow through the n-type region, which is at a voltage which facilitates the formation of a passivation layer (e.g., silicon dioxide), causes a passivation layer of silicon dioxide to be formed over the surface of the n-type region which stops the chemical etching. This leaves a thin silicon body of the desired thickness.
A problem with this type of electrochemical etching is that some reversed biased p-n junctions are much more leaky (i.e., they conduct current therethrough in the reverse direction) than others. This leakage is the result of the reverse resistance of the p-n junction which varies considerably from lot to lot. As a result, the voltage on the surface of the p-type region varies from lot to lot of p-n junctions. With current flow through the p-n junction and a sufficiently positive voltage on the etching surface of the p-type region, a passivation layer can form over the etching p-type region which cuts off further etching and therefore prevents the desired thickness of the semiconductor wafer from being achieved.
It is desirable to have an electrochemical etching technique for use with p-n junctions which is relatively insensitive to variations in reverse bias leakage through the p-n junctions.